1. Field of the Invention
This invention relates to a moire simulator for finding the screening conditions under which no moire pattern is generated due to interference between an original and a screen in a screening process for printing plate making and printing. This invention also relates to a method of using the moire simulator.
2. Description of the Prior Art
In printing plate making and printing, a screening process using a contact screen or a dot generator scanner is widely employed for converting the density of an original image to the magnitude of dot area and reproducing the original image as a printed image.
As described, for example, on pages 263 and 264 of "Insatsu Kogaku Benran" (Printing Engineering Manual) compiled by The Japanese Society Of Printing Science And Technology and published by Gihodo Shuppan K.K. on July 20, 1983, the screen angle for monochromatic image printing plate making is always 45.degree.. This is because, in the case of a monochromatic image, fringes become least perceptible when the screen angle is 45.degree.. In the case of color image printing plate making, the screen angle generally is adjusted to 45.degree. for a black print and 90.degree. for a yellow print when a process camera is used. Also, it is adjusted to 75.degree. (or 15.degree.) for a cyan print and 15.degree. (or 75.degree.) for a magenta print. When color scanners, i.e. electronic color separation devices, are used, the screen angles for color prints are adjusted to values different from those mentioned above, depending on the type of color scanner. For example, when Magnascan 520 supplied by Crosfield is used, the screen angle is adjusted to 15.degree. for a black print, 90.degree. for a yellow print, 75.degree. for a cyan print, and 45.degree. for a magenta print. When Scanagraph SG-808 supplied by Dainippon Screen Mfg. Co., Ltd. is used, the screen angle is adjusted to 90.degree. for a black print, 45.degree. for a yellow print, 15.degree. for a cyan print, and 75.degree. for a magenta print.
The reason why the screen angle for a yellow print is adjusted to 90.degree. in all cases is that, by positioning the dots of the least perceptible color in this direction, generation of the most perceptible abnormality of perpendicularly intersecting lines, for example, a moire pattern, can be decreased.
When a difference between the screen angles for two color prints is less than 15.degree., an optical interference fringe pattern called moire arises in halftone images. In order to eliminate this problem, the screen angles for a cyan print and a magenta print are adjusted so that the difference therebetween is within the range of 15.degree. to 30.degree.. In the case where a light magenta print or a light cyan print is additionally formed, the screen angle therefor is generally adjusted to 30.degree. or 60.degree..
In the case where a pattern having a grating constant, for example, a weave pattern of a woven fabric, a fine stripe pattern, a streak pattern of an exterior of a building or fittings, or a printed image constituted by dots, is present entirely or partially in an original image, fringes arise in a reproduced image due to interference between the grating-like patterns of the original image and the screen used in the screening process and make the reproduced image unsuitable for practical use when the angle between the directions of the grating-like patterns is less than 15.degree..
Particularly in color image printing, since color prints obtained by color separation and screening are used, a moire pattern readily arises due to interference between the screen and a grating-like pattern of an original image. As described on pages 439 and 440 of "Insatsu Kogaku Benran" mentioned above, generation of a moire pattern directly related to an original image including a grating-like pattern can be prevented by the methods described below.
(1) The direction of an ordinarily used screen is changed during exposure to light so that the angle between the grating-like pattern of the original image and the screen cells is 15.degree. or more.
(2) A thick plane-parallel glass or a polarizing prism is rotated at the front stage of a lens during exposure to light.
(3) An optical interference filter is positioned at a lens and exposure to light is conducted via the optical interference filter.
(4) A halftone printed original is copied to obtain a print, and an image of continuous tone is formed by the application of an air brush to the print. The simplest method is to carry out exposure to light by slightly deviating the focusing point of a lens.
The methods (2) and (3) mentioned above are not generally used in practice since they require a special optical device in addition to a process camera or a scanner for screening, and the cost of the apparatus becomes high or operations become complicated. The method (4) requires handicraft skills and is not suitable for use for general purposes since the original is limited to a halftone print.
Therefore, prevention of a moire pattern has heretofore been effected by using the method (1) and, when necessary, by deviating the focusing point of the lens.
In the aforesaid technique of preventing a moire pattern, when an unexpected moire pattern arises in a halftone separation image obtained by screening, the screening process is repeated by correcting the screen angle to obtain a screened image free from a moire pattern. Thus the technique involves a large loss of time and material. Further, in the color separation process and the screening process for color printing plate making, the screen angle for each separated color and the grating constant and direction of the grating-like pattern of an original image are related to each other in a complicated manner. Therefore, a moire pattern cannot be eliminated by a single correction of the screen angle but can be eliminated by repeating the screening process many times. Thus there is a marked loss of time and materials. Further, high-grade skill is required for doing the work correctly and quickly.